Halogenated 5,5-dialkylhydantoins (DAH) are common oxidizing biocides used for water sanitization. Examples of halogenated 5,5-alkylhydantoins include: dichlorodimethylhydantoin (DCDMH), dibromodimethylhydantoin (DBDMH), bromochlorodimethylhydantoin (BCDMH) and bromochloromethylethylhydantoin (BCMEH). The dialkylhydantoins are supplied in solid forms such as tablets or granules and are delivered to the system by water flow through a chemical erosion feeder charged with material. A combination of factors hamper the utility of this method. The rate of slug dosing is low because halogenated hydantoins are sparingly soluble in water (0.15-0.21%). The tablets and granules are often supplied as mixtures of chlorinated and brominated material since pure compounds are often difficult to produce. The different solubilities of the different compounds make accurate and reproducible delivery of oxidant difficult to attain. Also with different flow rates, the pressure drop across the chemical feeder varies, and this can produce non-uniform dissolution of the material. This also makes accurate and reproducible delivery difficult to attain.
Hypohalous acids (e.g., HOCl, HOBr) are also well known as oxidizing biocides for water treatment, and generally are generated in the liquid phase. Thus users avoid some of the limitations associated with solid chemicals. However, hypochlorous acid is unstable. Therefore, it is usually formed in-situ by treatment of the water with a precursor such as gaseous chlorine or sodium hypochlorite (bleach). Hypobromous acid is also unstable, and is formed in-situ by a number of methods. One method is the introduction of aqueous NaBr to the water, followed by activation with gaseous chlorine or bleach. Alternatively, stable perbromide (Br.sub.3.sup.-) solutions containing 30-40% Br.sub.2 are added to the water. When injected into the water system, the Br.sub.3.sup.- ion releases Br.sup.- ion and Br.sub.2. The latter immediately hydrolyses to HOBr and HBr according to the equation: EQU Br.sub.3.sup.- =Br.sup.- +Br.sub.2 =HBr+HOBr
In another method, bromine chloride is introduced which hydrolyses in the water to hypobromous and hydrochloric acids in accordance with the following equation: EQU BrCl+H.sub.2 O=HOBr+HCl
All the foregoing methods of generating and delivering hypohalous acids suffer a number of drawbacks. Gaseous chlorine, bromine chloride, and perbromide solutions possess high halogen vapor pressures. These pose serious storage and handling hazards, and are highly corrosive to metering and delivery equipment. In the water system, chlorine, bromine chloride and perbromide solutions release one mole of strong acid (HCl or HBr) per mole of hypohalous acid. Low local pH conditions are corrosive to metals. For perbromide solutions, only one of the three Br moieties of the Br.sub.3.sup.- ion materializes as HOBr, the other two are wasted. Sodium hypochlorite (bleach) is also unstable, and considerable amounts of NaOH are added to suppress deterioration. Therefore, application of bleach inadvertently increases the pH of the water and this can lead to local precipitation of metal hydroxides. The in-situ method of hypobromous acid generation by activation of NaBr with bleach or chlorine occurs under conditions of high dilution, and optimum control of the reaction stoichiometry is difficult. Underdosing of chlorine or bleach results in unreacted NaBr, while overdosing is a waste of material and can result in discharge limits being exceeded. In addition, for certain waters which are rich in ammonia and organic amines, the chlorine or bleach preferentially reacts to form stable chloramines which are unable to react with NaBr to form HOBr. Thus, NaBr is wasted.
Several of these limitations are addressed by an system in which accurately defined, higher concentrations of hypobromous acid are prepared and stored for subsequent delivery to the water system. In this method, aqueous NaBr, bleach and HCl are metered to a reaction tank to produce a solution containing 2500 ppm HOBr. The hydrochloric acid neutralizes the hydroxide ions introduced with the bleach. The pH is maintained at 2.5-3.0, since incomplete conversion of Br.sup.- ion to HOBr is claimed under neutral conditions. The weaknesses of this method include the following. Bleach is unstable and must be analyzed routinely. Adjustments to the metering equipment are required to compensate for the varying bleach quality. The metering and control equipment is expensive. The process uses three components: aqueous NaBr, bleach, and HCl.
In a related issue, hypohalite ions are known to disproportionate under alkaline conditions to produce halate ions: EQU 3XO.sup.- =2X.sup.- +XO.sub.3.sup.-
Thus, chlorate can be formed during the manufacture and storage of bleach (to which NaOH is deliberately added to supress deterioration). Bromate can be formed under conditions of high local pH during the activation of NaBr by bleach. Both bromate and chlorate may be present in alkaline water to which the corresponding hypohalous acid (or precursor) has been introduced. Halate ions are undesirable byproducts. They are not biocidally active and are a waste of hypohalous acid. Also, they are contaminants under consideration for regulation by the EPA.